Many different devices have been tried to protect combine harvesters from rocks. There are devices that include a door that is opened to allow rocks to drop to the ground. The presence of a rock must be sensed in some way by these devices and the door must be opened. Sensors often provide incorrect sensing or sensings that are not timely.
There are also devices which merely hold rocks until they are removed from the machine. These devices must include a holding area of adequate size, appropriate structure to force rocks into the holding area and the rocks must be retained in the holding area.